Polyhalite IMI Process For KNO3 Production

ABSTRACT

A process for producing KNO3 from polyhalite to is disclosed. In a preferred embodiment, the process comprises steps of (a) contacting polyhalite with HNO3; (b) adding Ca(OH)2 to the solution, thereby precipitating as CaSO4 at least part of the sulfate present in said solution; (c) precipitating as Mg(OH)2 at least part of the Mg2+ remaining in said solution by further addition of Ca(OH)2 to the remaining solution; (d) concentrating the solution, thereby precipitating as a sulfate compound at least part of the sulfate remaining in solution; (e) separating at least part of the NaCl from the solution remaining; and (f) crystallizing as solid KNO3 at least part of the K+ and NO3-contained in the solution. The process enables direct conversion of polyhalite to KNO3 of purity exceeding 98.5% and that is essentially free of magnesium and sulfate impurities.

REFERENCE TO RELATED PUBLICATION

This application claims priority from U.S. provisional application61/220,230, dated 25 Jun. 2009, which is hereby incorporated byreference in its entirety.

FIELD OF THE INVENTION

This invention concerns methods for production of KNO₃, in particular,methods that use polyhalite as the starting material.

BACKGROUND OF THE INVENTION

Potassium nitrate (KNO₃) is a commercially important chemical with usesfrom explosives to fertilizers. Polyhalite (K₂SO₄.MgSO₄.2CaSO₄.2H₂O) isa widely-distributed and readily available source of potassium. Reactionof polyhalite with acids (e.g. HNO₃) is known in the literature as ameans of producing crude salt mixtures containing “complex mineralfertilizer.” For example, U.S. Pat. No. 4,246,019 discloses a method forproduction of a. mixture containing 53.54% KNO₃, 39.87% Mg(NO₃)₂, 5.48%CaSO₄, and 1.01% H₂O from the reaction of polyhalite with HNO₃. RussianPat. No. 2,276,123 discloses a method for production of a solutioncontaining a mixture of K₂SO₄, MgSO₄ and NH₄NO₃ from reaction ofpolyhalite with HNO₃ followed by neutralizaion with ammonia. Thus theprocesses described in the literature allow to produce a solutioncontaining a mixture of substances, including potassium and magnesiumnitrates, as well as gypsum that had not been separated in early stagesof the process.

Several well-known processes, such as the Southwest Potash and IMIprocesses, are known for the production of KNO₃ by reaction of KCl withHNO₃ (Ullman's Agrochemicals, vol. 1; Weinheim: Wiley-VCH, 2007, pp.334-336). Production of essentially pure KNO₃ directly from the reactionof polyhalite with strong acid remains unknown, however. Thus, thereremains a long-felt need for a process that can produce essentially pureKNO₃ from polyhalite without the complications of known processes suchas production of complicated product mixtures and necessity ofneutralization with ammonia.

SUMMARY OF THE INVENTION

The process disclosed in the present invention produces KNO₃ frompolyhalite with almost total recovery, without any necessity forpreliminary thermal treatment and without the necessity for washing outof NaCl from the polyhalite. The separation of NaCl from the KNO₃ iseffected by the different temperature dependences of the solubility ofthe two substances: the solubility of NaCl changes very little withtemperature, while that of KNO₃ strongly increases with increasingtemperature. In the process herein disclosed, Mg is recovered asMg(OH)₂, which is precipitated with lime. The Mg(OH)₂ thus recovered canbe used directly as a slurry with water, dried or transformed into Mgsalts such as MgSO₄.H₂O.

It is therefore an object of the present invention to disclose a processfor producing KNO₃ wherein the starting material for the process ispolyhalite.

It is a further object of the present invention to disclose a processfor producing KNO₃ from polyhalite, comprising steps of (a) contactingpolyhalite with a substance comprising NO₃ ⁻; (b) adding at least oneinorganic base to the solution obtained in the step of contactingpolyhalite with HNO₃, thereby precipitating as a solid at least part ofthe sulfate present in said solution; (c) precipitating as Mg(OH)₂ atleast part of the Mg²⁺ remaining in said solution by adding at least onebasic compound to the remaining solution; (d) concentrating the solutionobtained after said step of precipitating at least part of the Mg²⁺remaining in said solution; (e) precipitating at least part of the NaClderived from said polyhalite, if any, from the solution obtained aftersaid step of concentrating the solution obtained after said step ofprecipitating at least part of the Mg²⁺ remaining in said solution; (f)separating said precipitated NaCl, if any, from the reaction stream, and(g) separating as solid KNO₃ at least part of the K⁺ and NO₃ ⁻ containedin the solution remaining after the step of precipitating at least partof the NaCl derived from said polyhalite. It is within the essence ofthe invention wherein said process is adapted to produce commerciallyusable KNO₃ from polyhalite.

It is a further object of this invention to disclose such a process,further including an additional step of washing said polyhalite prior tosaid step of contacting polyhalite with a substance comprising NO₃ ⁻,thereby removing at least a part of the NaCl contained within saidpolyhalite.

It is a further object of this invention to disclose such a process asdefined in any of the above, wherein said step of contacting polyhalitewith a substance comprising NO₃ ⁻ takes place at a temperature betweenabout 60° C. and about 90° C.

It is a further object of this invention to disclose such a process asdefined in any of the above, wherein said substance comprising NO₃ ⁻ ischosen from the group consisting of (a) HNO₃; (b) Ca(NO₃)₂; (c) anycombination of the above.

It is a further object of this invention to disclose such a process,wherein said substance comprising NO₃ ⁻ is HNO₃, and further whereinsaid step of contacting polyhalite with a substance containing NO₃ ⁻further includes an additional step of contacting polyhalite with aquantity of HNO₃ sufficient that the amount of HNO₃ in the solution thusobtained is at least 0.5% (w/w).

It is a further object of this invention to disclose such a process,wherein said substance comprising NO₃ ⁻ is HNO₃, and further whereinsaid step of contacting polyhalite with a substance containing NO₃ ⁻further includes an additional step of contacting polyhalite with aquantity of HNO₃ sufficient that the amount of HNO₃ in the solution thusobtained is at least 5% (w/w).

It is a further object of this invention to disclose such a process asdefined in any of the above, wherein said step of contacting polyhalitewith a substance comprising NO₃ ⁻ further includes an additional step ofcontacting polyhalite with 60% HNO₃.

It is a further object of this invention to disclose such a process asdefined in any of the above, further including an additional step ofrecycling into the reaction vessel at least part of the solutionremaining after said step of separating solid KNO₃.

It is a further object of this invention to disclose such a process,wherein at least a part of said substance comprising NO₃ ⁻ is obtainedfrom said solution recycled into said reaction vessel.

It is a further object of this invention to disclose such a process asdefined in any of the above, further including an additional stepremoving from the reaction stream at least part of the solid producedduring said step of contacting polyhalite with HNO₃.

It is a further object of this invention to disclose such a process,further including an additional step of removing by filtration at leastpart of the solid produced during said step of contacting polyhalitewith HNO₃.

It is a further object of this invention to disclose such a process,further including an additional step of washing said solid.

It is a further object of this invention to disclose such a process asdefined in any of the above, wherein said step of adding at least oneinorganic base to the solution obtained in the step of contactingpolyhalite with a substance comprising NO₃ ⁻ further includes anadditional step of adding at least one inorganic base chosen from thegroup consisting of Ca(OH)₂, CaCO₃, and CaO.

It is a further object of this invention to disclose such a process asdefined in any of the above, wherein said wherein said step of adding atleast one inorganic base to the solution obtained in the step ofcontacting polyhalite with a substance comprising NO₃ ⁻ further includesan additional step of adding sufficient inorganic base to reducesubstantially the SO₄ ²⁻ content of said solution.

It is a further object of this invention to disclose such a process asdefined in any of the above, wherein said step of adding at least oneinorganic base to the solution obtained in the step of contactingpolyhalite with a substance comprising NO₃ ⁻ further includes anadditional step of adding sufficient inorganic base to reduce the SO₄ ²⁻content of said solution by at least 85%.

It is a further object of this invention to disclose such a process,wherein said inorganic base is chosen from the group consisting of (a)basic Ca compounds; (b) basic Ba compounds; (c) any combination of theabove.

It is a further object of this invention to disclose such a process asdefined in any of the above, further including an additional step ofremoving from the reaction stream at least part of the insoluble sulfateproduced during said step of contacting polyhalite with HNO₃.

It is a further object of this invention to disclose such a process asdefined in any of the above, further including an additional step ofseparating by filtration at least part of the insoluble sulfate producedduring said step of contacting polyhalite with HNO₃.

It is a further object of this invention to disclose such a process asdefined in any of the above, wherein said step of precipitating asMg(OH)₂ at least part of the Mg²⁺ remaining in said solution furthercomprises an additional step of adding a sufficient amount of at leastone basic Ca compound to precipitate more than 50% of the Mg²⁺ remainingin said solution as Mg(OH)₂.

It is a further object of this invention to disclose such a process asdefined in any of the above, wherein said step of precipitating asMg(OH)₂ at least part of the Mg²⁺ remaining in said solution furthercomprises an additional step of adding a sufficient amount of at leastone basic Ca compound to precipitate more than 85% of the Mg²⁺ remainingin said solution as Mg(OH)₂.

It is a further object of this invention to disclose such a process asdefined in any of the above, wherein said step of precipitating asMg(OH)₂ at least part of the Mg²⁺ remaining in said solution furthercomprises an additional step of adding at least one basic Ca compoundchosen from the group consisting of Ca(OH)₂ and CaO.

It is a further object of this invention to disclose such a process asdefined in any of the above, further including an additional step ofremoving from the reaction stream at least part of said Mg(OH)₂ obtainedin said step of precipitating as Mg(OH)₂ at least part of the Mg²⁺remaining in said solution.

It is a further object of this invention to disclose such a process,further including an additional step of washing said Mg(OH)₂.

It is a further object of this invention to disclose such a process,wherein said Mg(OH)₂ is at least 92% pure.

It is a further object of this invention to disclose such a process asdefined in any of the above, wherein said step of concentrating thesolution remaining after said step of precipitating at least part of theMg²⁺ remaining in said solution further comprises a step chosen from thegroup consisting of (a) using a multiple effect evaporator toconcentrated said solution and (b) concentrating said solution bymechanical vapor recompression.

It is a further object of this invention to disclose such a process asdefined in any of the above, wherein said step of precipitating at leastpart of the NaCl further includes an additional step of precipitatingNaCl by evaporative crystallization.

It is a further object of this invention to disclose such a process,wherein said step of precipitating NaCl by evaporative crystallizationoccurs at a temperature exceeding about 60° C.

It is a further object of this invention to disclose such a process asdefined in any of the above, wherein said step of separating as solidKNO₃ at least part of the K⁺ and NO₃ ⁻ contained in the solutionremaining after said step of adding a basic Ca compound further includesan additional step of crystallizing KNO₃ from said solution.

It is a further object of this invention to disclose such a process,wherein said step of crystallizing KNO₃ from said solution furtherincludes an additional step of cooling said solution in order to affectcrystallization of KNO₃.

It is a further object of this invention to disclose such a process,wherein said step of cooling said solution includes a further step ofcooling said solution to a temperature below 40° C.

It is a further object of this invention to disclose such a process asdefined in any of the above, further including an additional step ofpurifying said KNO₃ obtained in said step of separating KNO₃.

It is a further object of this invention to disclose such a process,wherein said step of purifying said KNO₃ further includes an additionalstep of purifying said KNO₃ by at least one method chosen from the groupconsisting of (a) washing said KNO₃; (b) pulping with a substantiallypure KNO₃ solution; and (c) recrystallization.

It is a further object of this invention to disclose such a process asdefined in any of the above, wherein the purity of said KNO₃ exceeds98.5%.

It is a further object of this invention to disclose such a process asdefined in any of the above, wherein said step of precipitating at leastpart of the Mg²⁺ is carried out prior to said step of separating KNO₃.

It is a further object of this invention to disclose such a process asdefined in any of the above, wherein said step of precipitating at leastpart of the Mg²⁺ is carried out subsequent to said step of separatingKNO₃.

BRIEF DESCRIPTION OF THE FIGURE

FIG. 1 shows a schematic flowchart of the process herein disclosed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is described hereinafter with reference to thedrawings and examples, in which preferred embodiments are described. Forthe purposes of explanation, specific details are set forth in order toprovide a thorough understanding of the invention. It will be apparentto one skilled in the art that there are other embodiments of theinvention that differ in details without affecting the essential naturethereof. Therefore the invention is not limited by that which isillustrated in the figures and described in the specification, but onlyas indicated in the accompanying claims, with the proper scopedetermined only by the broadest interpretation of said claims.

In the detailed description that follows, formulas indicating water ofhydration are given according to conventional literature practice. Assuch, no claims are made regarding the specific level of hydration ofthe compounds (including those for which no water of hydration isindicated explicitly), and the invention herein disclosed is not limitedto the specific levels of hydration given.

The fundamental chemistry involved in the process herein disclosed canbe summarized as follows:

K₂SO₄.MgSO₄.2CaSO₄.2H₂O+2HNO₃+4H₂O+Ca(OH)₂[+Ca(NO₃)₂]→2KNO₃+Mg(NO₃)₂+4(CaSO₄.2H₂O)

The Mg(NO₃)₂ produced is precipitated as Mg(OH)₂ by further reactionwith Ca(OH)₂:

Mg(NO₃)₂+Ca(OH)₂→Mg(OH)₂+Ca(NO₃)₂

In the present invention, in contrast to methods known in the art,nitrate ion and calcium ion are added in separate independent steps ofthe process. In some embodiments of the process, Ca(NO₃)₂ recovered fromlater stages of the process is recycled and reacted with the polyhalite.

In a preferred embodiment of the invention herein disclosed, thefollowing series of steps is employed to produce a commercial grade ofKNO₃ from the reaction of polyhalite with HNO₃. The various “stages” arelisted with reference to the schematic flowchart given in FIG. 1 for apreferred embodiment 10 of the invention herein disclosed.

In some embodiments of the invention, NaCl is washed from thepolyhalite. This step is entirely optional; there is no requirement towash out the NaCl from the polyhalite, nor is there any need forpreliminary thermal treatment of the polyhalite.

In Stage 1 of the process (1010), polyhalite 101 is contacted with asubstance comprising NO₃ ⁻ 102. In preferred embodiments, substance 102is HNO₃; in most preferred embodiments, the HNO₃ is provided in 60%concentration. In preferred embodiments of the invention, a sufficientamount of 60% HNO₃ is added such that concentration of HNO₃ in theresulting solution (i.e. including the polyhalite) is at least 5% (w/w).In preferred embodiments of the invention, the polyhalite and HNO₃ arebrought into contact at a temperature of between about 60° C. and about90° C. In other embodiments of the invention, temperatures outside ofthe range used in preferred embodiments are used. In other embodiments,substance 102 may comprise another nitrate salt such as Ca(NO₃)₂, or amixture of a nitrate salt and HNO₃. In typical embodiments in whichCa(NO₃)₂ is used, it is added at the beginning of stage 1 in addition toor instead of the HNO₃ added at the beginning of the stage and the Cacompound added at the end of this stage, described below.

In preferred embodiments of the invention, at least part of the solution(210) obtained in stage 5 (described below) is recycled into thereaction vessel in which the contact between polyhalite and thesubstance comprising NO₃ ⁻ takes place.

The reaction mixture is then brought into contact with a base (inpreferred embodiments, a Ca(OH)₂ slurry (105)); in preferredembodiments, sufficient slurry is added to bring the pH to substantiallyneutral. Addition of the Ca(OH)₂ slurry thereby yields a solutioncomprising primarily K⁺, Mg²⁺, Ca²⁺, Na⁺, NO₃ ⁻, and Cl⁻, along withsolid CaSO₄ (gypsum). In preferred embodiments, sufficient base is addedto precipitate at least 85% of the SO₄ ⁻ present in the solution.

In preferred embodiments of the invention, solid gypsum precipitatedduring the reaction between the polyhalite and the HNO₃ is filtered andwashed with wash water 103 (in preferred embodiments, by counter-currentwashing on a filter 1015) in order to reduce the nitrate content. Themother liquor (106) is transferred to stage 2, while the wet gypsumafter washing (201) is discharged from the system. The wash filtrate 104is then returned to the reaction vessel in which stage 1 takes place.

In Stage 2 of the process (1020), additional Ca(OH)₂ slurry (105) isadded to the solution obtained in Stage 1 (106) after removal of solidgypsum in order to precipitate the major part of the Mg²⁺ contained inthe solution as Mg(OH)₂ (202); in preferred embodiments, sufficientCa(OH)₂ is added to precipitate at least 50% of the Mg²⁺ present. TheMg(OH)₂ is washed (1025) and removed. After precipitation of Mg(OH)₂, asolution comprising primarily Ca²⁺, K⁺, Na⁺, NO₃ ⁻, Cl⁻ and residualMg²⁺ remains. In some embodiments, this stage is carried out after Stage5 (described below) on the solutions to be recycled in stage 1. Thechemical purity of the Mg(OH)₂ produced is dependent on the purity ofthe CaO or Ca(OH)₂ used. In preferred embodiments, Mg(OH)₂ with a purityexceeding 92% is obtained.

The process then proceeds to Stage 3 (1030), in which the solutionobtained in Stage 2 is concentrated. In preferred embodiments, theconcentration is effected by evaporation using any technique known inthe art, e.g., a multiple effect evaporator or by mechanical vaporrecompression. In a preferred embodiment, at least part of the residualCaSO₄ thus precipitated is separated from the supernatant solution atthe exit of the vessel in which the concentration takes place. Theevaporation can be also carried out by solar evaporation in anevaporation pond and thus the calcium sulfate precipitated can be lefton the bottom of the pond.

The process then continues to Stage 4 (1040), in which NaCl (203) and asmall part of the CaSO₄ (204) present in the solution are partiallyseparated from the solution remaining after Stage 3 by crystallizationin an evaporative crystallizer at a temperature exceeding 60° C. Thesolids are separated (in preferred embodiments, by filtration 1045) andremoved.

In Stage 5 of the process (1050), KNO₃ is crystallized from the solutionby cooling the solution remaining from Stage 4. The crystallization canbe carried out by any technique known in the art, e.g., in a coolingcrystallizer of the various types existing, including cooling disccrystallizer. In typical embodiments of the invention, the purity of thewhite KNO₃ product obtained after washing in the tests exceeds 98.5%. Intypical embodiments of the invention, the main impurities are Ca(<0.2%), (<1000 ppm); Na (˜500 ppm); SO₄ ²⁻ (˜200 ppm); Mg (˜10 ppm);and Sr (˜10 ppm). The KNO₃ thus produced can be further purified by anytechnique known in the art, for example, by repulping with a pure KNO₃solution or by recrystallization.

The solution 210 remaining from Stage 5 is recycled to the vessel inwhich Stage 1 takes place. The Ca(NO₃)₂ contained in the solutionremaining from Stage 5 reacts with the sulfate in the solution in stage1 to precipitate gypsum.

EXAMPLE 1

Polyhalite (unwashed, crushed and screened to −0.5 mm, 400 g) was addedto a stirred mixture of nitric acid (59%, 146.7 g) and recycled solution(1090 g, made from combining mother liquor from KNO3 crystallizationpresented in Example 3 and gypsum wash water from a previous batch). Theconcentration of the nitric acid is modified by dilution with wash waterfrom previous runs in order to maintain a constant nitrate concentrationof 15-16% in the final filtrate. The reaction mixture was heated to 65°C. and stirred for 3 h. After that time, milk of lime (169.4 g, 30% inwater) was added dropwise via pump over a 1 h period to the hot mixturein order to neutralize the acidity of the slurry. When the mixturereached pH of 5.5-6.5 the addition was stopped and the mixture wasfiltered while hot under vacuum. The gypsum cake (700 g, 60.8% solids)was then washed with water (3×350 g) so that the nitrate content of thecake was satisfactorily low. The wet, washed gypsum (575.6 g, 73.9%solids) was then dried overnight in an oven at 60° C. yielding 425.5 gof gypsum (CaSO₄.2H₂O>98.5%, K<0.4%, Mg<0.2%, NO₃ ⁻ <100 ppm). Thefiltrate (1094.7 g, K=4.7%, Mg =1.5%, Ca=0.6%, SO₄ ²⁻=0.3%, NO₃ ⁻=15.5%)was used as the basis for the Mg(OH)₂ separation step (see Example 2below) while the wash water was combined with the recycled solution forthe next batch.

EXAMPLE 2

A sample of solution obtained after completion of the reaction presentedin Example 1 above (720-900 g of solution were treated at a temperatureof 60-70° C. with 15% solution of milk of lime (300 g). As a result ofthis treatment, the Mg concentration decreased from 1.5% to less than0.2%. The solids precipitated were settled and, afterwards, filtered andwashed. The dry solids contained more than 92% Mg(OH)₂. The mainimpurities were Ca (<5%), SO₄ ⁼ (2%), NO₃ ⁻ (0.2%) and Cl⁻ (0.05%).

EXAMPLE 3

A sample of the solution of remaining after the precipitation of Mg(OH)₂described in Example 2 above, comprising (concentrations on w/w basisrelative to the total solution) 2.2% Ca, 4.4% K, 1.9% Na, 0.01% Mg,13.3% NO₃ ⁻, 3.1% Cl⁻, and 0.08% SO₄ ²⁻ was concentrated by evaporationat a temperature exceeding 80° C. The total concentration of dissolvedsalts increased by >80% as a result of the concentration. The NaCl thuscrystallized was separated at a temperature exceeding 80° C. and itspurity after washing exceeded 98%.

The remaining solution was then cooled down to a temperature <40° C.,leading to precipitation of KNO₃, which was then separated from themother liquor and washed. Rhe purity of the KNO₃ obtained exceeded99.5%, while the concentration of dissolved salts in the mother solutionto be recycled to the reaction was in the range of 55-60%.

1-36. (canceled)
 37. A process for producing KNO₃ of at least 98%purity, wherein the starting material for said process is polyhalite andsaid process comprises: contacting polyhalite with a substancecomprising NO₃ ⁻; adding at least one inorganic base to the solutionobtained in the step of contacting polyhalite with a substancecomprising NO₃ ⁻, thereby precipitating as a solid at least part of thesulfate present in said solution; precipitating as Mg(OH)₂ at least partof the Mg²⁺ remaining in said solution by adding at least one basiccompound to the remaining solution; concentrating the solution obtainedafter said step of precipitating at least part of the Mg²⁺ remaining insaid solution; precipitating at least part of the NaCl derived from saidpolyhalite, if any, from the solution obtained after said step ofconcentrating the solution obtained after said step of precipitating atleast part of the Mg²⁺ remaining in said solution; separating saidprecipitated NaCl, if any, from the reaction stream; separating as solidKNO₃ at least part of the K⁺ and NO₃ ⁻ contained in the solutionremaining after the step of precipitating at least part of the NaClderived from said polyhalite; and, recycling into the reaction vessel atleast part of the solution remaining after said step of separating solidKNO₃.
 38. The process according to claim 37, further including anadditional step of washing said polyhalite prior to said step ofcontacting polyhalite with a substance comprising NO₃ ⁻, therebyremoving at least a part of the NaCl contained within said polyhalite.39. The process according to claim 37, wherein said step of contactingpolyhalite with a substance comprising NO₃ ⁻ takes place at atemperature between about 60° C. and about 90° C.
 40. The processaccording to claim 37, wherein said substance comprising NO₃ ⁻ is chosenfrom the group consisting of (a) HNO₃; (b) Ca(NO₃)₂; (c) any combinationof the above.
 41. The process according to claim 40, wherein saidsubstance comprising NO₃ ⁻ is HNO₃, and further wherein said step ofcontacting polyhalite with a substance containing NO₃ ⁻ further includesan additional step of contacting polyhalite with a quantity of HNO₃sufficient that the amount of HNO₃ in the solution thus obtained is atleast 5% (w/w).
 42. The process according to claim 37, wherein said stepof contacting polyhalite with a substance comprising NO₃ ⁻ furtherincludes an additional step of contacting polyhalite with 60% HNO₃. 43.The process according to claim 37, wherein at least a part of saidsubstance comprising NO₃ ⁻ is obtained from said solution recycled intosaid reaction vessel.
 44. The process according to claim 37, furtherincluding an additional step removing from the reaction stream at leastpart of the solid produced during said step of contacting polyhalitewith a substance comprising NO₃ ⁻.
 45. The process according to claim37, wherein said step of adding at least one inorganic base to thesolution obtained in the step of contacting polyhalite with a substancecomprising NO₃ ⁻ further includes an additional step of adding at leastone inorganic base chosen from the group consisting of Ca(OH)₂, CaCO₃,and CaO.
 46. The process according to claim 37, wherein said step ofadding at least one inorganic base to the solution obtained in the stepof contacting polyhalite with a substance comprising NO₃ ⁻ furtherincludes an additional step of adding sufficient inorganic base toreduce the SO₄ ²⁻ content of said solution by at least 85%.
 47. Theprocess according to claim 37, further including an additional step ofremoving from the reaction stream at least part of the insoluble sulfateproduced during said step of contacting polyhalite with HNO₃.
 48. Theprocess according to claim 37, wherein said step of precipitating asMg(OH)₂ at least part of the Mg²⁺ remaining in said solution furthercomprises an additional step of adding a sufficient amount of at leastone basic Ca compound to precipitate more than 85% of the Mg²⁺ remainingin said solution as Mg(OH)₂.
 49. The process according to claim 37,wherein said step of precipitating as Mg(OH)₂ at least part of the Mg²⁺remaining in said solution further comprises an additional step ofadding at least one basic Ca compound chosen from the group consistingof Ca(OH)₂ and CaO.
 50. The process according to claim 37, furtherincluding an additional step of recovering from the reaction stream atleast part of said Mg(OH)₂ obtained in said step of precipitating asMg(OH)₂ at least part of the Mg²⁺ remaining in said solution.
 51. Theprocess according to claim 50, further including an additional step ofwashing the Mg(OH)₂ recovered from the reaction stream.
 52. The processaccording to claim 50, wherein said recovered Mg(OH)₂ is at least 92%pure.
 53. The process according to claim 37, wherein said step ofconcentrating the solution remaining after said step of precipitating atleast part of the Mg²⁺ remaining in said solution further comprises astep chosen from the group consisting of (a) using a multiple effectevaporator to concentrated said solution and (b) concentrating saidsolution by mechanical vapor recompression.
 54. The process according toclaim 37, wherein said step of precipitating at least part of the NaClfurther includes an additional step of precipitating NaCl by evaporativecrystallization.
 55. The process according to claim 37, wherein saidstep of separating as solid KNO₃ at least part of the K⁺ and NO₃ ⁻contained in the solution remaining after said step of adding a basic Cacompound further includes an additional step of crystallizing KNO₃ fromsaid solution.
 56. The process according to claim 37, further includingan additional step of purifying said KNO₃ obtained in said step ofseparating KNO₃.